In the current state of the art, sports and other wide-area lighting applications typically utilize high intensity discharge (HID) lamps; most often, high wattage (e.g., 1000 watt or more), installed in a luminaire elevated high above the target area, and accompanied by a variety of optical devices which help to shape the light projected therefrom. Some typical optical devices used in HID luminaires include reflectors, lenses, visors, or the like and are designed to reflect, collimate, block, or otherwise direct light so to produce the desired beam pattern at or near the target area. In many applications the term “target area” refers not only to the surface where a task is performed, but also a defined space above and/or about said surface. As one example, the space above a baseball field could be considered part of the target area as it is desirable for a ball in flight to be appropriately illuminated throughout its trajectory.
HID lamps, and in particular metal halide HID lamps, are often the light source of choice because of a combination of long operating life (e.g., several thousand hours), high luminous output (e.g., over 100 k lm), high luminous efficacy (e.g., around 100 μm/W), excellent color rendering (e.g., CRI of 65 or more), and ability to mimic natural light (e.g., CCT around 4200K); the latter two features are particularly important for televised events. Over the years the art of designing wide-area HID lighting systems has evolved to address issues such as maintaining minimum light levels, ensuring specified lighting uniformities, and mitigating glare so to satisfy various safety, playability, or light pollution concerns, for example.
That being said, there is room for improvement in the art. For example, while high wattage HID lamps produce a significant amount of light, the lamps themselves are large (e.g., over 300 mm long and over 200 mm in diameter) and often require large and complex optical devices to harness the light and direct it towards the target area; this adds cost and size to the luminaire. Adding to the size of the luminaire often increases wind loading (i.e., drag) and weight; thus the elevating structure (e.g., pole) must be more substantial, which also adds to cost. Even then, there are limits to how much the light emitted from a single source can be shaped to suit a target area. For example, even with a host of optical devices, it is difficult for a single metal halide HID luminaire to adequately illuminate a bend in a road (e.g., as in a cloverleaf interchange) without spill (i.e., light that does not contribute to illumination of the target area and so is wasted).
One solution is to use several smaller (e.g., about 150 mm long and 75 mm in diameter), lower wattage (e.g., 400 watt) HID lamps in place of a single, high wattage HID lamp; this will presumably yield the benefits of HID lamps while potentially permitting a smaller, more compact luminaire with multiple light sources that can be independently controlled. Unfortunately, in the current state of the art lower wattage HID lamps suffer from reduced efficacy (e.g., around 80 lm/W). Given that many sports and other wide-area lighting systems are operated for twenty years or more before lamp replacement, the increased control does not justify the increased cost of operating the lower wattage HID lamps over time.
Light-emitting diodes (LEDs) are an attractive alternative light source because—given the appropriate operating conditions—they have a much longer operating life than HIDs (e.g., tens of thousands of hours) and an efficacy comparable to or exceeding HIDs; further, they can be designed to have a variety of color properties. A wide-area lighting system employing a plurality of LEDs has the potential to illuminate complex target areas in a manner not readily achieved using state-of-the-art HID lamps. That being said, the use of LEDs has not yet extended to sports and other wide-area lighting applications, at least in part, because simply swapping out one type of light source for another does not address the issue of heat management—a factor known to greatly impact the operating life and efficacy of LEDs—which, if not properly addressed, diminishes the benefits of using LEDs.
Another issue of great concern is “droop”—a phenomenon experienced by LEDs wherein efficacy sharply decreases as current increases. Droop is of particular concern for wide-area lighting applications—or any general lighting application—because high operating current is a necessity to make the use of LEDs more affordable. Unfortunately, the tradeoff is a significant decrease in efficacy; in some cases, increasing current beyond several milliamps (mA) results in a drop so severe as to render LEDs less efficient at converting electricity into light than other commercially available light sources (e.g., fluorescents). Further background regarding droop can be provided by a variety of sources including the following publication, the disclosure of which is incorporated by reference herein: “The LED's Dark Secret” [online], [retrieved 2011-07-13]/Retrieved from the Internet: <URL:http://spectrum.ieee.org/semiconductors/optoelectronics/the-leds-dark-secret/0>, published in IEEE Spectrum, vol. 46, issue 8, pp. 26-31 (2009).
Thus, there is a need in the art for sports and other wide-area lighting systems that capitalize on the benefits of LEDs while addressing heat management and droop, and yet, still prove cost-effective when compared to traditional HID systems. This is no easy task as it is estimated that an LED-based sports lighting system can cost several times as much (initially) as a standard HID-based sports lighting system; this is due, at least in part, to the sheer number of LEDs needed to approximate the light output of a single high wattage HID lamp.
One solution is to use LEDs capable of significant light output so fewer are needed to approximate the light output of a traditional HID lamp; presumably, this will increase the cost of the luminaire somewhat but permit greatly increased control of the light projected therefrom. The deficiency here is that because LEDs are still an emerging technology there is a limit to the light output that can be produced while maintaining an acceptable efficacy. Further, there is a limit to the size of optic that can be made to fit LEDs and still be formed by cost-effective molding techniques.
Another solution is to use commercially available LEDs driven at a higher than rated current; presumably, this will produce more light per LED so fewer are needed to approximate the light output of a traditional HID lamp. The deficiency here is that there comes a point when increasing current produces diminishing returns; droop and temperature increases, thereby reducing operating life and efficacy. Thus, there is room for improvement in the art.